In recent years there have been developed a variety of systems utilizing hydrogen absorbing alloys that can reversibly absorb and desorb hydrogen. Among them are, for example, heat utilization systems using the reaction heat involved in hydrogen absorption and desorption such as heat storage systems and heat pumps, and hydrogen absorbing systems which take advantage of such alloys to absorb a large amount of hydrogen.
Major necessary conditions for a hydrogen absorbing alloy to be used for such systems are that:
(1) The alloy can be easily activated in the initial stage of hydriding reactions;
(2) The alloy has a moderate hydrogen absorption and a desorption pressure at a given operating temperature;
(3) The difference between the hydrogen absorption and desorption pressures required for a reversible absorption-desorption process is small;
(4) The alloy has a great hydrogen absorption capacity.
(5) Inexpensive raw materials are available for the alloy;
(6) The alloy has an excellent durability for repeated hydrogen absorption-desorption cycles (i.e. a long cycle life).
There have been developed various hydrogen absorbing alloys for use with the hydrogen utilization systems as described above, which, depending upon the purpose and the conditions of the application, may be binary alloys such as rare earth-Ni alloys and Zn-Mn alloys, or multicomponent alloys formed from these alloys by partially substituting some of the compositions by some other elements (see for example Japanese Patents Laid Open Nos. 56-15772 and 56-29945; Journal of the Less-Common Metals, 53(1977)117-131)
Of these alloys, a family of rare earth-Ni alloys have been studied actively as promising candidates for the alloys satisfying aforementioned conditions (1) through (4). However, rare earth, which is a major composition, is very costly. In contrast, Zr-Mn alloys having Laves-phase structure are less costly compared with rare earth alloys, which have, however, greater differences between hydrogen absorption and desorption pressures due to hystereses in absorbing/desorbing a given amount of hydrogen, and greater plateaus slopes, i.e. the ratio (natural logarithmic change in pressure)/(change in the amount of the hydrogen absorbed [in wt %]) on the plateaus (which are those regions where the hydrogen absorption and desorption pressures remain substantially constant with the amount of the hydrogen absorbed and desorbed). Consequently, the differences in hydrogen absorption and desorption pressure required in reversible absorption and desorption are undesirably great for these alloys.
Further, with regard to the cycle life stated in Condition (6), Zr-Mn alloys suffer from a disadvantage that the amount of hydrogen absorbed and desorbed diminishes more significantly than rare earth-Ni systems and the hydrogen transfer rate decreases after repeated absorption-desorption processes. This may be attributed to the fact that the alloys are liable to pulverization at the time of hydrogen absorption and desorption and that alloying particles tend to coagulates with each other under the stresses due to the expansion and the contraction following the pulverization of the alloy, and consequently the hydrogen atoms absorbed in the alloy are prevented from smoothly flowing therein, thereby lowering the absorption/desorption reaction rate.
On account of this, the Zr-Mn alloys may greatly lower the efficiency of an application system such as a heat utilization system when used therein over a long period of time.